After hearing the gunshots, heroic Ryan Clark, 22, attempted to help Emily as she lay dying on the floor but was killed by Cho.

Cho then walked to Norris Hall carrying a backpack containing several chains, locks, a hammer, a knife, two handguns and nearly 400 rounds of ammunition.

He fired at least 174 rounds killing 30 and wounding 17.

A further six were injured when they jumped from windows in a bid to escape.

All of the victims were shot at least three times each, and 28 were shot in the head.

Sandy Hook – 27 killed

On 14 December 2012, mentally disturbed Adam Lanza, 20, shot and killed 20 children between the ages of six and seven at Sandy Hook elementary school in Newtown, Connecticut.

He also murdered six adult staff members, but as cops arrived at the school he shot himself dead.

Earlier he had shot and killed his mum, increasing the death toll to a horrific 27.

Lanza had used a 45 round/min semi-automatic assault rifle and a Glock pistol taken from his mother's extensive gun collection in her bedroom.

The brutal attack led to nationwide mourning and increased calls for tougher gun control, especially for deathly efficient automated weapons.

In an address to the media, President Barack Obama broke down in tears and vowed to double down in his efforts to tighten gun laws.

Texas church – 26 killed

During a Sunday service, on November 5, 2017, a man burst into a church in Sutherland Springs, Texas.

He opened fire using a semi-automatic weapon and gunned down multiple people.

At least 26 people were confirmed to have been killed and 30 others wounded.

23 people were shot inside the church and two were gunned down outside the building.

Another person died on the way to hospital.

Luby's massacre – 23 dead

The Luby's Massacre, also known as the Luby's shooting took place on October 16, 1991, at a Luby's restaurant in Texas.

Attacker George Hennard drove through the front window of the food venue in his pickup truck, and then shot and killed 23 people.

He also wounded over 20 others before fatally shooting himself.

San Ysidro McDonald's – 21 dead

This tragic event took place in and around a McDonald's restaurant in the San Diego neighbourhood of San Ysidro, California, on July 18, 1984.

Shooter James Huberty, 41, attacked and killed 21 people and injured 19 others, before he was shot dead by a SWAT team sniper.

He had previously suspected he had a mental health problem and had even called a mental health clinic requesting an appointment.

Marjory Stoneman Douglas shooting – 17 dead

On February 14, 2018, suspect and former student, Nikolas Cruz, wearing a gas mask and carrying smoke grenades, went on a rampage at his former school killing at least 17 people and injuring dozens more.

He pulled a fire alarm and gunned down students with an AR15 assault rifle as they evacuated Marjory Stoneman Douglas High School in Parkland, Florida.

The suspect, who was expelled in 2017 for unspecified "disciplinary reasons", managed to conceal himself within the crowds of terrified students fleeing the atrocity.

But police were able to use CCTV to identify the shooter and tracked him down to the nearby town of Coral Springs, where he was pinned down and arrested before being taken into custody.

Harrowing footage posted on social media showed terrified pupils crouching under their desks after they barricaded the doors as piercing gunshots were let off just feet away.

Twelve of the dead were killed inside the school, two others just outside, one more on the street and two other victims died of their injuries at a hospital, police said.

The victims included a mixture of adults and children, according to Broward County Sheriff Scott Israel who described the tragedy as "catastrophic".

Texas Tower Sniper – 17 dead

Charles Joseph Whitman, a former US Marine, shot dead 16 people in Austin, Texas, on August 1, 1966.

A seventeenth victim died 35 years later from injuries sustained in the attack.

He wounded 31 people.

Prior to the shooting Whitman had stabbed his wife and mother to death and written a suicide note.

Whitman was shot and killed by a police officer.

San Bernardino – 14 dead

In 2015, married couple Syed Rizwan Farook and Tashfeen Malik shot 14 people and injured 22 others in a mass shooting attack in San Bernardino, California.

The victims were attending a San Bernardino County Department of Public Health training event and Christmas party of about 80 employees.

Four hours after, police shot them dead in a shootout after pursuing their rented vehicle.

Wilkes-Barre – 13 dead

George Banks killed 13 people including five of his own children in Wilkes-Barre, Pennsylvania.

Walk of Death – 13 dead

Howard Barton Unruh, 28, shot and killed 13 people during a 12-minute walk through his neighborhood on September 6, 1949, in Camden, New Jersey. when he was 28 years old.

The incident became known as the "Walk of Death".

Unruh was an Army veteran who had seen extensive combat in Europe with the artillery in World War II.

He never stood trial after being diagnosed with paranoid schizophrenia.

Aurora – 12 dead

Mass murderer James Holmes slaughtered 12 innocent people after opening fire at a Colorado cinema in the summer of 2012.

The flame haired killer stormed a a midnight screening of Batman movie The Dark Knight Rises after legally purchasing an AR-15 assault rifle, a Remington 12-gauge shotgun, a .40 caliber Glock handgun and another pistol with 6,000 rounds of ammunition.

Shortly after the film started, the killer propped an emergency exit open with a table cloth holder, went to his car, changed into protective gear and a gas mask and retrieved his guns.

Holmes walked back into the theatre and threw two gas canisters into the audience.

Many in the audience thought it was a publicity stunt until he began spraying the crowd with the shotgun, then the assault rifle and finally the pistol.

A witness said he went outside and and shot people as they ran.

Cops apprehended Holmes in his car behind the cinema within minutes of the shooting.